A circuit of this type is often used for generating a regulated voltage for feeding other electronic components, which then form the load of the adapter circuit, inside a telephony device. The power used for generating the regulated voltage comes in this case from a telephone line to which the device is connected. The actual telephony device is often associated with an extra device called peripheral such as, for example, a telephone answering machine or a facsimile machine. Such a peripheral receives, in principle, the power it needs from a power grid and may thus form for the telephony device a better performing power supply source than the telephone line. It is thus desirable to provide the possibility of connecting the external power supply source formed by the peripheral to the load fed by the power supply adapter circuit. If the external power supply source delivers an output voltage having a higher value than the regulation value of the regulation module, the latter will attempt to lower this value without, however, managing it, since it is then the external power supply source that imposes the potential of the output terminal of the circuit. These attempts will, at best, cause a considerable waste of power, and, in the worst case, a breakdown of the external power supply source because of the very large consumption of power which the regulation module will spend on these vain correcting efforts.
It is an object of the present invention to remedy these drawbacks by proposing a power supply adapter circuit whose regulation module is inhibited when the output of the circuit is connected to an external power supply source which imposes on said output a voltage whose value is higher than the nominal regulation value.